Surface profile measuring apparatus, method of measuring surface profile, and method of manufacturing optical element

Abstract

A surface profile measuring apparatus includes a first image pickup device that obtains an interference pattern, an optical system that guides a measuring beam and a reference beam to the first image pickup device, a second image pickup device with which a distribution of light quantity of a beam from a light source traveling thereto avoiding the optical system is measured, and an arithmetic unit that calculates a profile of a target surface from the interference pattern. A distribution of light quantity of a beam from the light source transmitted through the optical system is measured with the first image pickup device. The profile of the target surface calculated by the arithmetic unit is corrected on the basis of the distributions of light quantity measured with the first and second image pickup devices.

What is claimed is: 
     
       1. A surface profile measuring apparatus in which a profile of a target surface is measured on the basis of an interference pattern formed by interference between a measuring beam reflected by the target surface and a reference beam, the apparatus comprising:
 a first image pickup device configured to pick up an image of the interference pattern; 
 an optical system configured to guide the measuring beam and the reference beam to the first image pickup device; 
 a second image pickup device with which a distribution of light quantity of a beam of the light from the light source traveling thereto avoiding the optical system is measured; and 
 an arithmetic unit configured to calculate a profile of the target surface on the basis of the image of the interference pattern picked up by the first image pickup device, 
 wherein a distribution of light quantity of a beam of the light from the light source transmitted through the optical system is measured with the first image pickup device, and 
 wherein the arithmetic unit is configured to obtain the profile of the target surface by calculating distortion of the optical system on the basis of the distribution of light quantity measured with the first image pickup device and the distribution of light quantity measured with the second image pickup device, and correcting, on the basis of the distortion, the profile of the target surface calculated on the basis of the interference pattern. 
 
     
     
       2. The surface profile measuring apparatus according to  claim 1 , further comprising:
 a beam splitter configured to split the light from the light source into two beams; and 
 a Fizeau lens having a Fizeau surface that splits one of the two beams into the measuring beam and the reference beam, 
 wherein a distribution of light quantity of the reference beam reflected by the Fizeau surface and transmitted through the optical system is measured with the first image pickup device, 
 wherein a distribution of light quantity of the other of the two beams is measured with the second image pickup device, and 
 wherein the profile of the target surface calculated by the arithmetic unit is corrected on the basis of the distribution of light quantity of the reference beam measured with the first image pickup device and the distribution of light quantity of the other beam measured with the second image pickup device. 
 
     
     
       3. The surface profile measuring apparatus according to  claim 2 , further comprising:
 a third image pickup device with which a distribution of light quantity of the measuring beam transmitted through the Fizeau surface is measured, 
 wherein the profile of the target surface calculated by the arithmetic unit is corrected on the basis of the distribution of light quantity of the measuring beam measured with the third image pickup device and the distribution of light quantity of the other beam measured with the second image pickup device. 
 
     
     
       4. The surface profile measuring apparatus according to  claim 2 , wherein the distribution of light quantity measured with the first image pickup device is corrected on the basis of at least one of a shape and a homogeneity of the Fizeau lens. 
     
     
       5. The surface profile measuring apparatus according to  claim 1 , further comprising:
 a first beam splitter configured to split the light from the light source into the measuring beam and the reference beam; 
 a reference mirror reflecting the reference beam from the first beam splitter; and 
 a second beam splitter provided in an optical path extending between the first beam splitter and the reference mirror and configured to split the reference beam into two beams, 
 wherein a distribution of light quantity of the measuring beam reflected by the target surface and transmitted through the optical system is measured with the first image pickup device, 
 wherein a distribution of light quantity of one of the two beams is measured with the second image pickup device, and 
 wherein the profile of the target surface calculated by the arithmetic unit is corrected on the basis of the distribution of light quantity of the measuring beam measured with the first image pickup device and the distribution of light quantity of the one beam measured with the second image pickup device. 
 
     
     
       6. The surface profile measuring apparatus according to  claim 5 , further comprising:
 a wavefront conversion element configured to convert a wavefront of the measuring beam from the first beam splitter and to guide the measuring beam to the target surface; and 
 a third image pickup device with which a distribution of light quantity of the measuring beam transmitted through the wavefront conversion element is measured, 
 wherein the profile of the target surface calculated by the arithmetic unit is corrected on the basis of the distribution of light quantity of the measuring beam measured with the third image pickup device and the distribution of light quantity of the one beam measured with the second image pickup device. 
 
     
     
       7. The surface profile measuring apparatus according to  claim 6 , wherein the distribution of light quantity measured with the first or third image pickup device is corrected on the basis of at least one of a shape and a homogeneity of the wavefront conversion element. 
     
     
       8. The surface profile measuring apparatus according to  claim 1 , wherein the distribution of light quantity measured with the first image pickup device is corrected on the basis of at least one of a shape and a homogeneity of an optical element included in the optical system. 
     
     
       9. The surface profile measuring apparatus according to  claim 1 , further comprising a drive unit configured to move the first or second image pickup device, wherein the drive unit moves the first or second image pickup device in at least one of a rotational manner and a translational manner. 
     
     
       10. A method of measuring a surface profile in which light from a light source is split into a measuring beam and a reference beam and a profile of a target surface is measured on the basis of an interference pattern formed by interference between the measuring beam reflected by the target surface and the reference beam, the method comprising:
 picking up an image of the interference pattern with a first image pickup device; 
 guiding, with an optical system, the measuring beam and the reference beam to the first image pickup device; 
 measuring, with a second image pickup device, a distribution of light quantity of a beam of the light from the light source traveling in such a manner as to avoid the optical system; 
 calculating, with an arithmetic unit, the profile of the target surface on the basis of the image of the interference pattern picked up with the first image pickup device; 
 measuring, with the first image pickup device, a distribution of light quantity of a beam of the light from the light source transmitted through the optical system; and 
 correcting the profile of the target surface calculated with the arithmetic unit on the basis of the distribution of light quantity measured with the first image pickup device and the distribution of light quantity measured with the second image pickup device. 
 
     
     
       11. The method of measuring a surface profile according to  claim 10 , further comprising exchanging the first image pickup device and the second image pickup device. 
     
     
       12. A method of manufacturing an optical element, comprising:
 measuring a profile of a surface of an optical element by the method of measuring a surface profile according to  claim 10 ; and 
 processing the surface of the optical element on the basis of a result of the measurement.